Universal Python venv wrapper

0 Python venv wrapper.md

$ ln -s /opt/work/my-env/../../../usr/local/bin/within-venv.sh /opt/within-my-venv && ln -s /opt/within-my-venv /usr/local/bin/mytool
$ mytool     # runs in /opt/work/my-venv

README.md

Usage

1. Create a virtual env in a directory. Example: python3 -m venv /opt/work/my-venv
2. Create a symbolic link to this script on your PATH, in a special way:

• The source of the link must be this script, as a relative path to the virtual environment to run within.
• The name of the link should be an executable file within the bin/ directory of the virtual environment. Example: ln -s /opt/work/my-env/../../../usr/local/bin/within-venv.sh /usr/local/bin/mytool
• Alternatively, create a "wrapper link" from the relative path to this script from the virtual environment. Then, create a link named after your target executable to this link. Example: ln -s /opt/work/my-env/../../../usr/local/bin/within-venv.sh /opt/within-my-venv && ln -s /opt/within-my-venv /usr/local/bin/mytool

What's Happening

You are running this script. The script then runs mytool (or your link name) after adding your virtual environment's bin/ directory to the PATH. It determines the correct PATH by inspecting the special way you created the link.

within-venv.sh

#!/bin/bash
# Copyright Maxwell Bloch 2019
# Licensed GPL version 2 or later

unwind() {
    path=${1}
    path=$(dirname ${path})
    while [ "$(basename ${path})" != ".." ]; do
        path=$(dirname ${path})
    done
    while [ "$(basename ${path})" = ".." ]; do
        path=$(dirname ${path})
    done
    echo ${path}
}

if [ -L ${0} -a -f ${0} ]; then
    script="$(basename ${0})"
    wrapper=${0}
    dirname=$(readlink -f $(dirname ${wrapper}))
    while [ -L ${wrapper} ]; do
        wrapper=$(readlink ${wrapper})
	if [ "$(echo ${wrapper} | cut -c1)" != "/" ]; then
	    wrapper="${dirname}/${wrapper}"
	fi
    done
    path="$(unwind ${wrapper})"
fi

if [ -x ${path}/bin/${script} ]; then
    PATH="${path}/bin:$PATH" ${path}/bin/${script} "$@"
else
    echo "Script or program '${script}' not found in path ${path}/bin"
    exit 1
fi